Both batt cables on a 4104 connect to bulkhead bolts in the batt compartment, it is kind of difficult to figure out how they properly connect.

It does sound as if he has two cables connected in reverse due to the low voltage after trying to crank.

jj,

Please check very carefully that you don't have your batts connect in reverse, it sounds that way to me. the huge drop in voltage makes me think this. This is easy to do in a 4104 because all those cables look like a spider's nest and the two bulkhead connectors are easy to mix up.

If you're charging the batts separately then connecting them this is probably what is happening. However, if you're charging them while connected in parallel that may not be the problem. I'm not sure what would happen if you charge two together connected in series??

Another thing that could cause that huge drop in voltage is a stuck starter but I can't explain why it works a while later. A stuck starter will draw huge amps and voltage and won't crank the engine.

Corroded connections could cause some voltage drop but I don't think that much. I once cleaned all my cable connections from the batt to the starter and it did help quite a bit, those cables are pretty long and old.

Another thing to check is the starter relay in the rear right hand elect comp but that wouldn't explain the huge voltage drop. A bad relay will keep the starter from cranking but won't drop the voltage because the circuit isn't activated.

One other thought and I'll stop guessing, it could be a cable that is so frayed only a few strands are left and when you try to crank they get really hot and increase resistance. A badly corroded connection will do the same thing. Then after it cools it will once pass current for a short time. A wild guess but possible!

The batteries are at 12.6v or above. I did have a problem with switching the pos and neg cable connections once upon a time, but I think I have that figured out. I am worried about the bad connections thing. I extended the wiring harness' and moved the panel to behind the driver's seat for easier access. I used solid wire. A friend of mine who is an electrical engineer told me after the fact that that may have been a bad idea as stranded and solid wire sometimes don't mix well. I know the 4104 has a lot of redundancies and protection circuits, but I am guessing there is not a lot of actual 'electronics' that go into the operation of the motor and drive system itself. Pretty much mechanical in 53' right? So, I may have to wire directly into the starter and engine electronics to bypass the panel. Because at this point, I have no idea where the bad connection might be. A good portion of the wire is dry-rotted and corroded . And I have no access to the tunnel running down the middle and cannot trace wires through there. A little depressing, but I knew I was in for a challenge when I bought a 60 year old bus with millions of miles on it.

This is all standard 4104 stuff/problems. Get a good smart charger and your bats will live longer. Buy batteries with a guarantee and you may save enough to fix other stuff, Poor grounds have not been mentioned but you need to start there. Until you KNOW your electrical system is fully functional, do not leave the batteries connected while away from the vehicle as a short can cause a fire and you need to br around to put it out. Speaking of which make sure you have a fairly large fire extinguisher in/on/near the bus. For trouble shooting wiring I use a couple of extension cords and alligator clips to make sure circuits are getting the volts they need. When I use the original wiring I always isolate it and check it for continuity and then for voltage drop to make sure it is usable. Finally one group 31 bat will start your engine, so keep it simple for now as having the 2 bats connected just hides problems and can lead to a bad bat killing the other bat. BTW master switch and starting relays MS relay almost definitely need to be serviced/replaced, especially if the are original, I use a lot of cube relays on my bus as the huge old relays, while still functional, are very inefficient and leak a lot of voltage even after PM/adjusting.

Okay. I don't understand all the weird voltage stuff that was happening. But it appears it was just a case of a ruined battery. It's running again! Thankfully it was under warranty. I think I need to go trade the other one in too.

You probably connected the two pos to neg and ruined one, lucky you didn't get both. Knowing the 4104 as I do I assure you this is not hard to do. I've had a few batts ruined when a mate shorted out.

You will continually have elect problems from corroded wires, especially at the terminal ends at the panel stud connectors. I cleaned those terminals, replaced the nuts with brass, replaced a bunch of the terminals and things got better. You will find that those old steel nuts and posts really caused the brass terminals and copper wires to corrode - too many different kinds of metal in contact for too many years. I cut about an inch off every wire when I replaced the terminals, went back to fresh clean wire.

You may not be a mechanic now but you will be after you own it a few years, it is a good project for training in many skills.

There is nothing electronic in a 4104 except maybe you could consider the selenium rectifiers electronic but that is a stretch! All that I could see they were used for was to isolate alarms so the circuits could be multi-purpose.

Zub is right, just use a single Gp31 or 29 batt for now so you won't have this problem anymore until you get it on the road.

These things happen. Just yesterday I remove both my 4D starts for their two year cleaning and refilling. I found one 2/0 ground cable completely corroded off, it was solid green. It was broken but snug up against the lug so the break wasn't obvious-you never know! Luckily the PO had tied the two batt ground posts together so the batt was still in the circuit. Otherwise I would have been operating on one batt and not have known it.

I may stick with 4Ds after all since the posts are on the end like the 8D. With my 4107 batt box design side posts would be a pain to access. It would also require changing to longer cables-I may be too lazy to do all that!